Contactless power systems comprise of a contactless power transmitter that includes a conductive path supplied with alternating current from a power supply and one or more contactless power receivers. These contactless power receivers are within proximity, but electrically isolated from, the conductive path. A contactless power receiver includes a pick up coil in which a voltage is induced by the magnetic field generated by the conductive path, and supplies an electric load. The pick up coil is usually tuned using a tuning capacitor to increase power transfer capacity of the system.
One of the issues with contactless power receivers is their low efficiency when they are lightly loaded, for example when a motor powered by a power receiver is idle while it awaits a command from a control system. This can be overcome by implementing power flow control via a power controller between the pick-up coil and the load.
One implementation of a power controller uses a shorting switch as part of the pick-up circuit to decouple the pick-up coil from the load as required. This approach is described in the specification of U.S. Pat. No. 5,293,308 assigned to Auckland UniServices Limited and is referred to as “shorting control”.
Although the technology addresses the power flow control problem from the pickup to the load, the shorting switch can cause large conduction losses, especially at light loads because the pickup circuit is nearly always shorted under no load or light load conditions.
Another problem with contactless power systems is frequency variations due to changes in load conditions and other circuit parameters. This can cause changes in the pick-up coil in terms of the induced voltage magnitude and short circuit current, which affect the power transfer capacity of the system. This is particularly a problem in fixed or passively tuned contactless power receivers.
One approach described in US patent specification US2007/109708A1 & U.S. Pat. No. 7,382,636B2 is to dynamically tune or de-tune the power pick-up by varying the effective capacitance or inductance of the power receiver. This enables the contactless power receiver to compensate for frequency drifts caused by parameter changes. The effective capacitance or inductance is varied by employing two semiconductor switches in series with the capacitor or inductor. Also a means of sensing pick-up coil current magnitude and phase is required to enable soft switching of the variable capacitor or resistor. By implementing dynamic tuning not only can frequency drifts be compensated for but much higher quality factors (Q>10) can be realized than in passively tuned systems (normally Q<6) as the pick-up coil resonant frequency can be fine tuned. Higher quality factor increases the power transfer capacity of the systems.
In order to miniaturize the contactless power pickup circuitry it is beneficial to eliminate the pick-up coil sensor which is particularly complicated at high frequencies. Removal of this eliminates the ability to soft switch the variable capacitor or inductor i.e. the system is now hard switched. This implementation causes excessively high currents or voltages because either the inductor current can be switched off or the capacitor can be shorted during the switching process. The resulting switching transients contribute to EMI, stresses of tuning semiconductor switches, and reduces the system power efficiency due to excessive power losses. In the worst cases it can cause system failure.
It is an object of the present invention to provide a method for controlling, or apparatus for contactless power receivers which will ameliorate one or more of the disadvantages suffered by existing systems, or which will at least provide the public with a useful alternative.